"As a society we recognise that non-smokers need to be protected from carcinogens when at work but we are not doing enough to protect the most vulnerable non-smokers of all - children," he said.

Amanda Sandford, research manager for Action on Smoking and Health (ASH), called for a smoking ban in all public places.

She added: "The best thing parents can do for the health of themselves and their children is to stop smoking."

Simon Clark, director of the smokers' lobby group Forest, said: "The effects of passive smoking are notoriously difficult to measure.

Most studies are based on imprecise recall and anecdotal evidence concerning the exact amount of exposure to environmental tobacco smoke. Yet this report, like so many, adopts a preposterous pretence of precise measurement which immediately arouses suspicion.

"To isolate the effect of environmental tobacco smoke on lung cancer cases would require an examination of all possible alternative causes.

"Unfortunately it is just another example of anti-smoking hysteria, a further attempt to demonise smokers for their habit."